The present invention relates to electrically driven pumps in general, and in particular to a pressure regulated electrically driven double acting reciprocating pump.
Circulation systems are often used to deliver a liquid coating material such as paint to coating stations for application onto articles to be coated. A paint circulation system customarily comprises a pump for the paint, motor means for operating the pump, and a paint flow line that extends from an outlet from the pump, past the various coating stations to which paint is to be delivered and back to an inlet to the pump. Each coating station is connected to the paint flow line for receiving paint upon demand by coating application equipment at the station, with any paint not provided to a coating station being circulated through the paint flow line and returned to the pump inlet, whereby paint not delivered to a coating station is circulated and maintained in motion so that pigments and fillers in the paint remain in suspension.
Since coating application equipment often has flow characteristics that are pressure dependent, for it to operate properly it usually is necessary that coating liquid or paint be delivered to it at a substantially constant pressure. A goal of paint circulation systems is therefore to provide paint at a constant pressure to the painting equipment, irrespective of the flow rate of paint demanded from the pump. The flow demand that the pump must meet has an absolute minimum that is based upon the minimum flow velocity required to keep paint pigments and fillers in suspension. As coating or paint stations go "on" or "off" the flow demand rises and falls at levels above the absolute minimum. Changes in flow demand tend to result in changes in pump outlet pressure.
Two types of supply pumps commonly used in paint circulating systems are turbine pumps which are kinetic pumps and reciprocating pumps which are positive displacement pumps. An advantage of a turbine pump is that it has a very flat pressure response over a wide range of flow rates, which enables the pump to provide a generally constant pressure paint flow under changing flow demands. This is particularly useful in painting systems where flow characteristics are pressure dependent, but there are two significant disadvantages of turbine pumps. One is that while a turbine pump is typically driven by an induction type motor having a relatively high efficiency in the 85% to 90% range, the efficiency of the pump itself is very low, usually on the order of 25% to 40%. The other disadvantage is that the constant "slip" of the liquid being pumped, against the walls of the impellers and bowls, degrades the pigments and fillers that are suspended in the paint. The worst case of paint degradation occurs when a turbine pump is running full speed with all painting stations "off." Turbine pumps are seldom speed controlled, so slip and churning of the paint are at a maximum when there is no demand for paint by the coating stations.
Positive displacement double acting reciprocating pumps utilize a piston to pump paint, and as compared to turbine pumps have the advantage of being nonaggressive to and causing minimal degradation to pigments and fillers in the paint, and of being able to attain higher operating efficiencies. In addition, unlike a turbine pump a reciprocating pump does not run at full speed all the time. Reciprocating pumps are driven by sources that operate under the principal of balancing forces caused by the driving pressure and the driven pressure, so they run at a minimum speed when all coating stations are "off" and speed up only as flow demands increase. Reciprocating pumps normally have relatively high efficiencies in the 85% to 90% range, but they can be and customarily are driven by rociprocating air or hydraulic mechanisms that have relatively low efficiencies on the order of about 20% and 60%, respectively. In addition to reducing the overall efficiency of the paint circulation system, there are other disadvantages to reciprocating air and hydraulic driving mechanisms. In the case of a reciprocating air driving mechanism, freezing problems can and do occur due to the rapid expansion of the exhausted air at changeovers, which occur at changes in direction of the reciprocating air driving mechanism. Air dryers can aid in reducing the freezing problem by taking moisture out of the air, but dryers can be a large capital expense and reduce overall system efficiency by requiring additional power. As for hydraulic driving mechanisms, they have the disadvantage of potentially serious oil contamination of the paint being pumped.
To avoid the disadvantages of air and hydraulic mechanisms for driving reciprocating pumps, electric motors have been used for the purpose, and a crank and connecting rod or a cam and cam follower have been utilized to convert the rotary output of the motor to the reciprocating motion of the pump. However, the effort has brought with it its own unique disadvantages, since both crank and connecting rod, and cam and cam follower, converting mechanisms result in a serious problem in maintaining a constant pump outlet pressure at changeover of the pump, i.e., during the time when the direction of reciprocation of the pump is reversed. As the reciprocating pump. approaches changeover, both types of converting devices result in a rapidly decreasing load torque on the electric motor that allows the motor to rapidly speed up to account for the decreasing reciprocating velocity of the pump relative to the somewhat constant rotational speed of the motor. Also, at changeover the checks or check valves that control entry and exit of liquid to the pump reverse position, which has the effect of "catching" the rapidly rotating motor and severe shocks can result. Then, immediately after changeover the decreased load torque abruptly changes to rapidly increasing load torque that causes the electric motor to rapidly slow down. The net effect is a situation with difficult to control pressure drops and pressure spikes that respectively occur just before and just after changeover.